Advanced Neutron Focusing Optics for Pulsed Sources: Development and Prospects

The evolution of neutron scattering from reactor-based steady-state sources to high-power pulsed spallation sources has necessitated a paradigm shift in neutron optics. While pulsed sources offer high peak brilliance and energy-resolved measurements via the time-of-flight (TOF) technique, the intrinsic divergence and broad wavelength bandwidth of the incident beam pose significant challenges for focusing, particularly in the realm of very small-angle neutron scattering (VSANS, Q < 0.001 Å−1). This review presents a comprehensive analysis of diverse focusing techniques, including converging multi-slit apertures, electrical and superconducting magnetic sextupole lenses, grazing-incidence focusing mirrors, compound refractive lenses with oscillation apertures, and a special multi-beam VSANS configuration. Special attention is given to the transition from permanent magnet systems to nested rotating sextupole permanent magnets (Nest-Rot-SPM) and modulated superconducting sextupoles (SSM), detailing the physical and engineering challenges involved. Furthermore, grazing-incidence reflective optics, notably toroidal Wolter mirrors, are discussed as an achromatic alternative. The integration of these technologies into world-leading pulsed neutron sources is reviewed to project the future landscape of extended Q-range coverage for SANS instruments.